Quantitative and qualitative aspects of platelet production will be studied. Megakaryocyte numbers and volumes are believed to reflect the ongoing rate of platelet production. On a volume basis megakaryocytes are larger than other marrow cells, with little overlap. An electronic particle analysis system of sufficiently wide range has been to distinguish and measure megakaryocytes. Excess marrow from routine aspirates will be analysed in a large number of patients; the accumulated data will permit clinical correlations and inferences about pathophysiologic principles, and should generate usable concepts to aid clinicians to interpret megakaryocytes in clinical marrow examinations. A number of coagulation factors are intimately associated with platelets. Their functions in blood clotting are unknown, but information on their origins and locations would be important. They have been thought to be absorbed from plasma, but some evidence suggests they may be intrinsic. The origins of these proteins will be investigated by attempting to demonstrate their synthesis in megakaryocytes. Isolated megakaryocytes (96% pure) will be cultured with radioactive amino acids so newly labelled proteins could be identified by specific antibodies to the factors. The anatomic localization of these antigens in intact and aggregated platelets will be determined ultra structurally by a sensitive immunohistochemical technique.